The plasma display technology now makes it possible to achieve flat colour panels of large size and with limited depth without any viewing angle constraints. The size of the screens may be much larger than the classical CRT picture tubes would have ever allowed.
Plasma Display Panel (or PDP) utilizes a matrix array of discharge cells, which could only be “on” or “off”. Therefore, unlike a Cathode Ray Tube display device or a Liquid Crystal Display device in which gray levels are expressed by analog control of the light emission, a PDP controls gray level by a Pulse Width Modulation of each cell. This time-modulation is integrated by the eye over a period corresponding to the eye time response. The more often a cell is switched on in a given time frame, the higher is its luminance or brightness. Let us assume that we want to dispose of 8 bit luminance levels i.e. 255 levels per color. In that case, each level can be represented by a combination of 8 bits with the following weights:                1-2-4-8-16-32-64-128        
To realize such a coding, the frame period can be divided in 8 lighting sub-periods, called sub-fields, each corresponding to a bit and a brightness level. The number of light pulses for the bit “2” is the double as for the bit “1”; the number of light pulses for the bit “4” is the double as for the bit “2” and so on . . . . With these 8 sub-periods, it is possible through a combination to build the 256 gray levels. The eye of the observers integrates over a frame period these sub-periods to catch the impression of the right gray level. The FIG. 1 shows such a frame with eight sub-fields.
The light emission pattern introduces new categories of image-quality degradation corresponding to disturbances of gray levels and colors. These is defined as “dynamic false contour effect” since it corresponds to disturbances of gray levels and colors in the form of an apparition of colored edges in the picture when an observation point on the PDP screen moves. Such failures on a picture lead to the impression of strong contours appearing on homogeneous area. The degradation is enhanced when the picture has a smooth gradation, for example like skin, and when the light-emission period exceeds several milliseconds.
When an observation point on the PDP screen moves, the eye follows this movement. Consequently, it no more integrates the same cell over a frame (static integration) but it integrates information coming from different cells located on the movement trajectory and it mixes all these light pulses together, which leads to a faulty signal information.
Basically, the false contour effect occurs when there is a transition from one level to another with a totally different sub-field code. The European patent application EP 1 256 924 proposes a code with n sub-fields which permits to achieve p gray levels, typically p=256, and to select m gray levels, with m<p, among the 2n possible sub-fields arrangements when working at the encoding or among the p gray levels when working at the video level so that close levels have close sub-field codes i.e. sub-field codes with close temporal centers of gravity. As seen previously, the human eye integrates the light emitted by Pulse Width Modulation. So if you consider all video levels encoded with a basic code, the temporal center of gravity of the light generation for a sub-field code is not growing with the video level. This is illustrated by the FIG. 2. The temporal center of gravity CG2 of the sub-field code corresponding to a video level 2 is superior to the temporal center of gravity CG3 of the sub-field code corresponding to a video level 3 even if 3 is more luminous than 2. This discontinuity in the light emission pattern (growing levels have not growing gravity center) introduces false contour. The center of gravity of a code CG(code) is defined as the center of gravity of the sub-fields ‘on’ weighted by their sustain weight:
      CG    ⁡          (      code      )        =                    ∑                  i          =          1                n            ⁢                        sfW          i                *                              δ            i                    ⁡                      (            code            )                          *                  sfCG          i                                    ∑                  i          =          1                n            ⁢                        sfW          i                *                              δ            i                    ⁡                      (            code            )                              where—sfwi is the sub-field weight of ith sub-field;                δi is equal to 1 if the ith sub-field is ‘on’ for the chosen code, 0 otherwise; and        SfCGi is the center of gravity of the ith sub-field, i.e. its time position.        
The center of gravity SfCGi of the seven first sub-fields of the frame of FIG. 1 are shown in FIG. 3.
So, with this definition, the temporal centers of gravity of the 256 video levels for a 11 sub-fields code with the following weights, 1 2 3 5 8 12 18 27 41 58 80, can be represented as shown in FIG. 4. As it can be seen, this curve is not monotonous and presents a lot of jumps. These jumps correspond to false contour. The idea of the patent application EP 1 256 924 is to suppress these jumps by selecting only some levels, for which the gravity center grows smoothly. This can be done by tracing a monotone curve without jumps on the previous graphic, and selecting the nearest point.
Such a monotone curve is shown in FIG. 5. It is not possible to select levels with growing gravity center for the low levels because the number of possible levels is low and so, if only growing gravity center levels were selecting, there will not be enough levels to have a good video quality in the black levels since the human eye is very sensitive in the black levels. In addition the false contour in dark areas is negligible. In the high level, there is a decrease of the gravity centers. So, there will be a decrease also in the chosen levels, but this is not important since the human eye is not sensitive in the high level. In these areas, the eye is not capable to distinguish different levels and the false contour level is negligible regarding the video level (the eye is only sensitive to relative amplitude if we consider the Weber-Fechner law). For these reasons, the monotony of the curve is necessary just for the video levels between 10% and 80% of the maximal video level.
In this case, 40 levels (m=40) are selected among the 256 possible levels. These 40 levels permit to keep a good video quality (gray-scale portrayal). This is the selection that can be made when working at the video level, since only few levels, typically 256, are available. But when this selection is made at the encoding, there are 2n different sub-field arrangements, and so more levels can be selected as seen on the FIG. 6, where each point corresponds to a sub-field arrangement (there are different sub-field arrangements giving a same video level).
The main idea of this Gravity Center Coding, called GCC, is to select a certain amount of code words in order to form a good compromise between suppression of false contour effect (very few code words) and suppression of dithering noise (more code words meaning less dithering noise).
The problem is that the whole picture has a different behavior depending on its content. Indeed, in area having smooth gradation like on the skin, it is important to have as many code words as possible to reduce the dithering noise. Furthermore, those areas are mainly based on a continuous gradation of neighboring levels that fits very well to the general concept of GCC as shown on FIG. 7. In this figure, the video level of a skin area is presented. It is easy to see that all levels are near together and could be found easily on the GCC curve presented. The FIG. 8 shows the video level range for Red, Blue and Green mandatory to reproduce the smooth skin gradation on the woman forehead depicted on the FIG. 7. In this example, the GCC is based on 40 code words. As it can be seen, all levels from one color component are very near together and this suits very well to the GCC concept. In that case we have almost no false contour effect in those area with a very good dithering noise behavior if there are enough code words, for example 40.
However, let us analyze now the situation on the border between the woman forehead and the woman hairs as presented on the FIG. 9. In that case, we have two smooth areas (skin and hairs) with a strong transition in-between. The case of the two smooth areas is similar to the situation presented before. In that case, we have with GCC almost no false contour effect combined with a good dithering noise behavior since 40 code words are used. The behavior at the transition is quite different. Indeed, the levels required to generate the transition are levels strongly dispersed from the skin level to the hair level. In other words, the levels are no more evolving smoothly but they are jumping quite heavily as shown on the FIG. 10 for the case of the red component.
In the FIG. 10, we can see a jump in the red component from 86 to 53. The levels in-between are not used. In that case, the main idea of the GCC being to limit the change in the gravity center of the light cannot be used directly. Indeed, the levels are too far each other and, in that case, the gravity center concept is no more helpful. In other words, in the area of the transition the false contour becomes perceptible again. Moreover, it should be added that the dithering noise is also less perceptible in strong gradient areas, which enable to use in those regions less GCC code words more adapted to false contour.
So a solution is to select locally the best coding scheme (in terms of noise/dynamic false contour effect trade-off) for every area in the picture. In this way, the gradient based coding disclosed in the European patent application EP 1 522 964 can be a good solution to reduce or remove the false contour effect when the video sequence is coded by a gravity center coding of EP 1 256 924. The idea is to use a “normal” gravity center coding for areas that have a smooth gradation (low gradient) in the signal level, and a reduced set of codes (=a subset of the set of normal gravity center codes) for the areas that undergo a high gradient variation in the signal level (transition). A reduced set of codes comprising 11 code words is for example shown in FIG. 11. This reduced set has an optimal behaviour in terms of false contour for these regions but the regions where it is applied must be carefully selected in order to not introduce dithering noise. The selection of the regions where the reduced set of codes is applied is made by a gradient extraction filter. FIG. 12 shows the gradient regions detected by a gradient extraction filter in the picture of FIG. 7. The high gradient regions are displayed in white in this figure. The other regions are displayed in black.
So the gradient based coding disclosed in EP 1 522 964 is considered as a good solution to reduce the dynamic false contour effects in the different areas or regions of the picture. But, it remains some dynamic false contour effects on the boundary between two areas (i.e. between an area coded by codes of a reduced set (high gradient) and an area coded by codes of a “normal” set (low gradient)). Dynamic false contour effects are introduced due to the shift between the two sets of codes. This is mainly due to a non optimal selection of the boundary position where the two neighbouring pixels are coded with two different codes that are not fully compatible even if coming from the same skeleton.